1. Field of the Invention
The present invention relates to filtration systems. Filtration systems have conventionally been used for either (1) removal of particulate matters from fluid suspensions to result in clear, non-turbid fluids or, (2) removal and discarding of part of the soluble and fluid fraction for the purpose of concentrating the desirable particulate matters. To achieve the first purpose, the filter in the system is used to trap the particulate matters by virtue of the effective pore sizes being smaller than the particulate matters, while allowing the soluble fraction to go through the filter pores and collected for subsequent use. To achieve the second purpose, the ideal filter will allow the soluble fraction to go through the filter pores with only minimal entrapment of the particulate matters which are then returned to collection containers as the "retentate" fraction for subsequent use. In both procedures, clogging of the filter remains a major problem.
2. Description of the Prior Art
The clogging of filter pores is a major problem with prior art filtration and dialysis devices. Clogging of the filter pores quickly reduces the efficiency of the filtration system. As the number of unclogged pores diminishes, filtration rate decreases. Since flow rate is equal to pressure gradient divided by resistance, as more and more filter pores are clogged (increasing resistance), a progressively larger pressure gradient is needed to maintain adequate flow rates. Even then, when enough of the filter pores become clogged, flow rate will become for all practical purposes, zero. At that point, particulate matters can no longer be removed from fluid suspensions. In addition, if the purpose is to concentrate suspended particulate matters, clogging of filters will decrease the final yield of the particulate matters and may in fact decrease the concentration of such matters in the retentate.
To minimize the problem of clogging, various approaches have been designed, as reflected in different filtration systems on the market. One approach incorporates designs for vigorous stirring of the suspension physically above, or prior to interaction of the suspension with the filter surface. Examples include the Stirred Cells Series of Amicon Division, W.R. Grace & Co. However, such systems are ineffective because the distance between the stirring mechanism and the filter membrane (typically in millimeters) are several orders of magnitude larger than the diameter of the particles (typically in microns). Once the particulate matters are trapped within the filter pores, with constant positive filtration pressure pressing them against the filter membrane, agitation at a far distance (relative to the size of the particulate matters) will not effectively dislodge them. Moreover, high shearing forces generated by vigorous stirring may cause foaming and denaturation of the particulate matters.
Another approach involves the concept of tangential flow as exemplified by Millipore's Minitan system. Instead of applying pressure perpendicular to the surface of the filter, the suspension is pushed forward by positive pressure from a pump system so that it travels in a direction tangential to the filter surface. In theory, this design allows the particulate matters to travel in a direction tangential to the filter surface while the soluble phase goes through the filter pores in a direction perpendicular to the filter surface. In practice, however, substantial clogging still occurs. The reason is that the particulate matters are carried by the soluble phase of the suspension and will travel in the direction of the immediate fluid surrounding a given particle. Any time a fraction of the soluble phase goes through the filter pores (in a direction perpendicular to the filter surface), a proportional amount of particulate matters will travel with it in the same direction. Regardless of the direction of flow of the rest of the suspension bulk (which may travel in a direction tangential to the filter surface), the fraction that goes through the pores will clog up the pores. With this understanding, it becomes clear that tangential flow filter systems are only different ways of recirculating the bulk of the suspension before its interaction with the filter pores. This design does not substantially alter the clogging potentials of particulate matters at the level of the filter pores because the particulate matters are again pressed onto the pores by the positive pressure used to circulate the bulk of the suspension.
Since both the stirred cell design and the tangential flow systems use positive pressure to circulate the suspension, they both result in trapping of particulate matters within the matrix of the filter membrane. For this reason, these systems are not suitable for the purpose of concentrating particular matters. There exists a need for a novel design where: (1) the filter membrane will not be clogged, and (2) should unexpected change in the filtration condition lead to some clogging, the obstructed pores will become unclogged again. Such a device will allow efficient concentration of valuable particulate matters. In addition, because of the increased life span of the filter membrane, it also allows cost-efficient collection of the soluble phase of the suspension, if the soluble phase is the desirable fraction from the suspension.
In special filtration systems such as ultrafiltration for handling ultrafine particles suspended in a fluid phase, the filter membrane is often anisotropic, consisting of a thin skin supported by a porous backing. As such, the skin is the barrier which separates the retentate from the permeate and does not necessarily have microscopically visible pores. However, positive pressure forcing the fluid out from the retentate through the skin to become the permeate promotes the accumulation of particles next to the skin which impedes further improvement in filtration efficiency. This is known as the gel resistance. The greater the positive pressure to squeeze the fluid out of the skin, the greater is the gel resistance. When the filtration efficiency (defined as "flux" which is often in units of gallons of permeate obtained per square foot filter area per day, or GSFD) is plotted against transmembrane pressure (pressure difference between the retentate surface and the permeate surface of the filter), initially flux increases with increase in transmembrane pressure. Rententate surface is defined as that surface of the filter which faces the retentate or particulate suspension. The permeate surface is the other surface of the filter which faces the permeate or filtrate fluid. Particles are defined here as materials carried by the fluid phase to be separated by the filter membrane. Particles can therefore be in a dissolved state, or colloidal or solid state. However, after a certain transmembrance pressure is obtained, flux increases no more and becomes a constant value. This is due to the increase in resistance which builds up as transmembrane pressure is increased. Very often, due to the large particle sizes as compared to the sub-microscopic pore sizes of the skin, the filter is not described as being clogged, but is described technically as having resistance (e.g. gel resistance) that impedes further increase in flux. This technicality is recognized and will be included in this application as part of the "clogging" process since clogging is defined here as any process that decreases the filtration efficiency of the filtration system.
To overcome the problem of clogging, examples in the prior art include the following:
1. Pre-filters are used to pre-sieve large particles and to decrease the total load presented to the main filter. Pre-filters have, by necessity, pore sizes larger than the pore sizes of the main filters. This method does not prevent the clogging of either the main filter or the prefilter. This method is actually two filtration systems working in tandem or in sequence with all the classical problems associated with such systems. Once some pores become clogged, the number of pores available for filtration decreases and the particles are often trapped inside the filter matrix and not recirculated back to the retentate.
2. Tangential flow is an attempt to decrease the clogging of filter pores by introduction of a sweeping force tangential to the surface of the membrane instead of perpendicular to it to sweep away any clogging material. This is performed by pushing the fluid in a direction tangential to the membrane surface instead of directly onto it (perpendicularly to it). However, such systems inevitably use positive pressure to squeeze the fluid out of the filter. Positive pressure is defined here as pressure within the filtration system where particles are pushed onto the retentate surface of the filter. Positive pressure can be created by either a pump upstream from the filter system forcing the suspension onto the retentate surface to "squeeze" the fluid out as permeate, or by a vacuum system downstream from the permeate surface of the filter "sucking" the permeate out. To the extent that the fluid entering the pores is travelling in a direction perpendicular to the surface of the filter, the particles carried by that portion of the fluid will also travel in a direction perpendicular to the surface of the filter and therefore will clog the pores. The remaining portion of suspension (in fact the bulk of suspension) that does not interact with the pores of the filter will not clog the pores, and therefore it does not matter what direction it travels as far as clogging is concerned. Therefore, tangential flow is only a method of moving the bulk of suspension which has no immediate effect on clogging and does not solve the problem of clogging at the level of the pore sites.
3. Back flushing--many filtration systems are obligated to terminate the filtration process when the filtration efficiency drops below acceptable levels. Instead of discarding the expensive filters, a fresh fluid medium is forced from the permeate surface of the filter toward the retentate surface to back-flush out the material that clog the pores. This method has several disadvantages: (a) back-flushing time means down-time for the filtration process; (b) back flushing is inefficient. Let us consider two clogged pores; pore X is 99% clogged and pore Y is only 10% clogged. At a given back-flushing pressure, pore Y is going to be unclogged first. After pore Y becomes unclogged, the back-flushing fluid can go through pore Y with little resistance. Since fluids tend to travel in the pathway of least resistance, there is now even less pressure to unclog pore X once pore Y has been unclogged. Therefore, the pores that need most unclogging get the least unclogging pressure; (c) Since the particles are trapped during the filtration process, they do not recirculate back to the retentate. If the purpose of the filtration is to increase the concentration of the particles, trapping of the particles within the matrix of the filter defeats that purpose; (d) Some particles are pressure-sensitive. Continued impaction within the pores of the filter may irreversibly damage the particles.
The present inventor, Dr. Richard C. K. Yen has filed two presently co-pending patent applications concerning filtration systems. They are as follows:
1. U.S. patent application Ser. No. 07/292,991 filed 01/03/89 entitled "Anti-Clogging And Dialysis Device For Filtration Systems".
2. U.S. patent application Ser. No. 07/311,345 filed 02/16/89 entitled "Vacuum Suction Type Anti-Clogging And Dialysis Device For Filtration Systems".
The anti-clogging devices described in the co-pending applications utilize a negative pressure that constantly unclogs any pores that are about to clog while filtration goes on. Negative pressure is defined here as any pressure in the filtration system that promotes movement of particles away from the retention surface of the filter. This can be achieved by either a pump downstream from the retentate surface of the filter "pulling" the particles away from the retentate surface of the filter, or a pump pushing fluid from the permeate surface of the filter toward the retentate surface.
These prior art applications have several distinguishing features: (a) the anti-clogging process is an on-going continuous process. It is not a two step approach of conventional filtration to be followed by back-flush; (b) the anti-clogging process is efficient. It has the greatest anti-clogging potential where clogging is most likely to occur in the filtration system; (c) anti-clogging method can be used on particles that are pressure-sensitive because the pores do not trap the particles; (d) all the pores are continuous being cleansed and therefore the filter has the maximal number of functional pores and can achieve the maximal filtration efficiency at all times.
It was stated in the pending applications that volume-regulated pumps are preferred. This is to ensure that whatever pressure that the pump generates to either recirculate the retentate or to extract the permeate, some constant flow of either permeate or retentate can be measured and has values greater than zero. A discussion of the ratio of recirculation flow rate versus permeate (filtrate) extraction rate recognized that for different suspensions, this ratio may vary. It was emphasized that the flow rate of the retentate must be greater than the flow rate of permeate extracted from the suspension in order to generate a negative pressure always greater than the positive pressure pushing the particles against the retentate surface of the filter. The present invention involves an improvement on this process to modify the principles disclosed and claimed in the copending applications for use with other types of suspensions.